1. Field of the Invention
The present invention relates to surface light sources used for devices such as liquid crystal displays, and particularly to a surface light source having a light guide plate with diffusion structures arranged thereon in order to provide uniform illumination.
2. Description of the Prior Art
In general, LCDs have two main advantages in comparison with cathode ray tubes (CRTs): LCDs are thin, and have low power consumption. It has been said that LCDs might one day completely replace CRT display devices, and LCDs have aroused great interest in many industries in recent times. In general, an LCD needs a surface light source to provide uniform light for a clear display.
A surface light source comprises a feed light source and a light guide plate. The feed light source may be a linear light source, or one or more point light sources. The light guide plate has an incident surface through which light from the feed light source is introduced, and an emitting surface for emitting light.
The light guide plate functions to change a direction of propagation of light beams emitted from the feed light source. The light is changed from a direction roughly parallel to the emitting surface of the light guide plate to a direction roughly perpendicular to the emitting surface. That is, the light guide plate effectively changes the linear or point light source(s) into a surface light device, for evenly illuminating a whole display screen of the LCD. Thus, the performance of the surface light source greatly depends on the characteristics of the light guide plate employed therein.
FIG. 3 shows a conventional surface light source 1, which comprises a reflector 12, a light guide plate 13, a diffuser 14, a brightness enhancing film (BEF) 15, and two point light sources 11. The reflector 12, the light guide plate 13, the diffuser 14 and the BEF 15 are stamped together in that order from bottom to top. The point light sources 11 are disposed adjacent an incident surface 131 of the light guide plate 13. The BEF 15 has a plurality of parallel prisms 151 evenly disposed on a top surface (not labeled) thereof In use, light beams from the point light sources 11 enter the light guide plate 13 through the incident surface 131, and then are introduced into the BEF 15 in a direction perpendicular thereto. The BEF 15 converges the light beams to evenly illuminate an associated LCD panel.
FIG. 4 shows an optical intensity distribution of essential light beams in the light guide plate 13. FIG. 5 shows an optical intensity distribution of essential light beams of either one of the point light sources 11. Each point light source 11 emits light beams over a limited predetermined range of angles, and the light beams enter the light guide plate 13 with an uneven distribution. As a result, three dark areas 261,262,263 are created in the light guide plate 13. The luminance of the dark areas 261,262,263 is less than that of other areas. That is, light beams exiting from the surface light source 1 are not uniform.
In addition, the surface light source 1 requires a plurality of elements, including the diffuser 14 and the BEF 15 for enhancing brightness. This adds to the size and the cost of the surface light source 1.
A new surface light source and a light guide plate for the surface light source which overcome the above-mentioned disadvantages are desired.